Chain stitch sewing machines of such types basically incorporate the advantage that they can be run at comparably high RPM-rates without any problems. A basic problem however at such type of sewing machine does exist as to such drive the looper as to achieve reliable conditions at the instant of the needle thread loop pick-up and at the instant when the descending needle enters a thread triangle for the double thread chain stitch formation.
German Offenlegungsschrift No. 33 13 981 describes a looper drive mechanism for a sewing machine of the aforesaid type incorporating a stitch length adjustable workpiece feed mechanism. At this machine basically the path described by the tip of the looper remains unaltered, however this path as a total is such as displaced that the distance between the tip of the looper and a vertical line represented by the needle positioned in its lower dead center remains always almost constantly. This oscillating at constant amplitude is such as induced that at increasing of the stitch length resp. feed rate the initial point of the path described by the tip of the looper will be moved away from the described vertical line. At a reduction of the stitch length resp. of the feed rate the initial point of the path described by the tip of the looper will be moved towards the described vertical line.
U.S. Pat. No. 3,285,210 also describes a looper drive mechanism for a chain stitch sewing machine of the aforesaid type, at which the looper is oscillatable at a constant amplitude. At stitch length resp. feed rate alterations the path of the looper is such as displaced at maintaining its constant amplitude in the feed direction transversal to the latter, so that about a constant distance is achieved between the tip of the looper and the vertical line represented by the needle positioned in its lower dead center.